When determining the effectiveness of the removal of a composition from a solid surface, typically a standardize amount of composition on a standardized solid surface is used in order to compare the effectiveness between two or more removal techniques. A variety of different contaminants contacting a variety of different surface materials have been used as standards for cleaning. Of particular past interest has been in the field of containment and decontamination strategies for the recovery process following an aerosol distribution of hazardous substances, particularly an urban radiological dispersion device (RDD) or a so-called “dirty bomb”. The physical impact of an RDD in a given area is a function of the explosive design, the radioactive material type, and weather (e.g. rain and wind). The size and shape of the contaminated area is dependent upon the geometry of the area, the device geometry, the size distribution of the RDD particles and the wind conditions after the explosion. Cleanup after an RDD will likely occur for weeks to months after the event; therefore the contaminated area will be exposed to a variety of weather conditions (rain, snow, relative humidity (RH) variation, etc). This may allow penetration of the water-soluble radioactive material into permeable surfaces increasing the difficulty of removing the contaminant.
Previously, a contaminant was placed in liquid form on the surface, allowed to dry and then used as a standard for measuring the effectiveness of a specific cleaning or removal technique. However, such deposition of contaminant onto a surface is itself not standardized and suffers from uneven adsorption of liquid into the surface, uneven drying, liquid spreading on the surface and uneven surface penetration (particularly for porous surfaces). Also, liquid deposition is not ideal for a standardized representation of aerosol deposition of contaminants onto surfaces. Furthermore the surface will need to be horizontal when adding the liquid, which does not reflect many normally occurring aerosol contamination events.
Aerosols have been used to coat various surfaces to prepare a standardized surface. This has been done using a particle-settling chamber. This chamber contains the target surface(s) at the bottom and particles are introduced into the chamber top. Particles are continuously mixed with air to generate a homogenous mixture for deposition. Deposition occurs by settlement of larger particles onto the target surface(s). This arrangement suffers from lack of standardization because different sized particles or droplets settle at different rates and aerosol diffusion can generate uneven deposition. The method is also time consuming when high surface concentrations are required.
The method is also dependant on the initial particle concentration and the rate of later, additions, air dilution or the mixing method and rate. Differing mixing, diffusion and dilution methods will also cause differing amounts of particle deposition on the sides of the particle settling chamber, thereby preventing that subset of particles from depositing on the target surface(s). Still further, if the particles do not deposit onto the sides of the particle-settling chamber, a greater concentration of certain sized particles that have inelastic collisions with the side walls may be deposited on bottom surfaces adjacent to the side walls and not uniformly over the target surface(s).
Small particles (e.g. less than one micron in diameter) may remain suspended for a very long time (particularly when agitated) and are deposited based on diffusion whereas larger particles are deposited based on gravity caused settling. Very light (low density) particles and charged particles have a similar problem with remaining in suspension for a very long time. Such deposition is hard to control, being based on different properties for different sized or types of particles. Furthermore, the use of a settling chamber does not reflect many real-life aerosol contamination situations.
To overcome these problems, the following invention deposits aerosols onto target surfaces in an easy and controlled manner, which yields a more uniform and standardized test surface sample. Such surfaces provide a better standard control for decontamination of and evaluation of coated surfaces.